Spark-Ignition (SI) combustion engines typically consume a significant amount of fuel during activation and idle operation. Various methods including Belt Alternator/Starter (BAS) systems and hybrid electric drivetrain systems have been proposed to reduce fuel consumption. Combustion-assisted start/stop operation involves trapping a fuel/air charge that is sufficient to produce starting torque in at least one cylinder during engine deactivation.
During activation, cylinders containing the trapped fuel/air charge in the proper position are ignited to rotate a crankshaft of the engine. The resulting motion positions subsequent cylinders of the engine for combustion. In order to accomplish combustion-assisted starting, the fuel/air charge must be sufficient to produce starting torque in at least one cylinder having a piston positioned after a Top Dead-Center (TDC) position of a compression stroke and before a Bottom Dead-Center (BDC) position of an expansion stroke.
In one approach, combustion-assisted starting is implemented in a direct-injection gasoline SI engine with a conventional valvetrain system. To enable combustion-assisted starting, the following shutdown sequence is performed in chronological order. First, an Electronic Throttle Control (ETC) adjusts a Manifold Absolute Pressure (MAP) of the vehicle to provide an air flow rate that is required to produce starting torque of the engine. Second, a sufficient amount of fuel to produce the starting torque is injected into some of the cylinders. Third, the ignition systems of the cylinders containing a trapped fuel/air charge are deactivated. Fourth, the engine is deactivated so that the crankshaft comes to rest between one-half and one revolution after BDC of an intake stroke of the first cylinder in sequence with a trapped air/fuel charge.
However, if the crankshaft comes to rest during a compression stroke of the first cylinder in sequence, the opportunity to start the engine with the crankshaft rotating in the proper direction is lost. Additionally, if the crankshaft comes to rest during the exhaust stroke of the first cylinder in sequence, the unburned fuel/air charge is discharged to the exhaust system. This eliminates the possibility for combustion-assisted starting and compromises vehicle emissions.